James Stevens, author of the Yachtmaster Handbook, spent 10 of his 23 years at the RYA as Training Manager and Yachtmaster Chief Examiner

I am no diesel fitter but even I can detect the hollow sound a marine diesel makes when the cooling system is short of salt water. It’s usually a more noticeable sign of trouble than the lack of water coming out of the exhaust. If you ignore it, the heat alarm will go off and then it’s definitely time to turn off the engine. The most common reason is that the seawater inlet seacock is turned off. Next is a damaged impeller, possibly due to a blocked inlet seacock.

The impeller pumps seawater through the engine cooling system. Normally the pump’s casing is cold to touch but if the impeller fails and water stops pumping through, it feels hot.

The new impeller and its washer will need lubrication, and in this case it’s been supplied

At this point you need a spare impeller, a screwdriver, some lubricant like silicone grease, glycerin or washing up liquid, a small box for the screws and some kitchen roll. It’s a pretty quick job if you’ve done it before. First, close the seawater inlet seacock, undo a few screws to remove the cover plate using kitchen roll to catch any drips, take out the damaged impeller, lubricate the new one and insert.

On a moving boat those screws are going to find their way to the deepest part of the bilge if you don’t keep them safe. You have to lubricate the new impeller and ensure the blades are bent over the right way for when it starts turning. Replace the cover with the new washer, also lubricated, open the seawater inlet seacock and check for any leaks when you restart the engine.